1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More particularly, the present invention is in the field of fabrication of heterojunction bipolar transistors.
2. Related Art
GaAs-based devices are able to provide the power and amplification requirements of wireless communication applications with improved linearity and power efficiency. Of particular interest are gallium-arsenide (xe2x80x9cGaAsxe2x80x9d) heterojunction bipolar transistors (xe2x80x9cHBTxe2x80x9d), which exhibit high power density capability, making them suitable as low cost and high power amplifiers in devices used in CDMA, TDMA and GSM wireless communications. The NPN gallium-arsenide HBT has significant advantages in speed, frequency response, and breakdown when compared to a conventional silicon bipolar transistor. The higher breakdown, speed and frequency response of the gallium-arsenide NPN HBT are possible due to certain advantages of gallium-arsenide, such as a large band gap and high electron mobility. Gallium-arsenide has a bandgap of 1.424 eV compared to 1.1 eV for silicon, and its electron mobility is on the order of 5000-8000 cm2/(V-sec) as compared to 800-2000 cm2/(V-sec) for silicon. As a result, GaAs devices can achieve a significantly greater gain bandwidth product and breakdown at comparable frequencies and input powers to silicon devices.
It is known that when an NPN gallium-arsenide HBT is inactive, a space charge layer is present at the P-N, collector-base junction, due to the diffusion of majority carriers across the P-N interface. Without applied voltage, the width of the space charge layer, i.e. the xe2x80x9cdepletion regionxe2x80x9d, is relatively narrow. Further, the electric field across the depletion region resulting from the migration of the majority carriers is relatively weak. However, when a reverse voltage is applied to the collector-base junction, i.e. when the collector-base junction is reversed biased, the electric field is strengthened, and the depletion region is correspondingly widened, with the edge of the depletion region extending further and further into the collector. The electric field strength and the collector depletion region width are directly related to the applied reverse voltage.
It is further known that electrons injected from the emitter into the base are accelerated across the depletion region by the electric field to the collector where they are xe2x80x9ccollected.xe2x80x9d The strength of the electric field across the depletion region determines the energy of the electrons traveling through the collector-base junction. Therefore, if the reverse voltage applied to the junction is raised, the electric field is strengthened, and the energy of the electrons increases correspondingly.
At a certain voltage referred to generally as xe2x80x9cbreakdown voltage,xe2x80x9d many of the electrons traveling across the collector-base junction attain sufficient energy that through collisions, electron-hole pairs are generated in the depletion region. The collision-created holes and electrons contribute to the reverse current, i.e. the current flowing from collector to base, and a very rapid increase in the reverse current results. Provided that the device""s maximum power dissipation limits are not exceeded, the xe2x80x9cbreakdownxe2x80x9d which occurs as a result of the rapid increase in the current does not necessarily lead to catastrophic failure of the HBT. However, the heat generated and the resulting rise in temperature could threaten the operation of the device.
Referring now to FIG. 1, an NPN gallium-arsenide HBT fabricated utilizing conventional fabrication methods is illustrated. Gallium-Arsenide (xe2x80x9cGaAsxe2x80x9d) HBT 100 comprises emitter contact 120, base contacts 122 and 124, and collector contact 126. Further, GaAs HBT 100 comprises emitter cap 118, emitter cap 116, emitter 114, and base 112. In a typical GaAs HBT, emitter cap 118 is indium-gallium-arsenide (xe2x80x9cInGaAsxe2x80x9d) doped with tellurium at about 1xc3x971019 cmxe2x88x923, for example, while emitter cap 116 is gallium-arsenide doped with silicon at approximately 5xc3x971018 cmxe2x88x923. Emitter 114 can comprise either aluminum-gallium-arsenide (xe2x80x9cAlGaAsxe2x80x9d) or indium-gallium-phosphide (xe2x80x9cInGaPxe2x80x9d) doped with silicon but at the relatively low concentration of 3xc3x971017 cmxe2x88x923. Base 112 can be, for example, gallium-arsenide doped with carbon at typically 4xc3x971019 cmxe2x88x923.
Continuing with FIG. 1, as shown, GaAs HBT 100 further comprises collector 130 and subcollector 110. According to conventional fabrication methods, collector 130 comprises gallium-arsenide, which is uniformly and lightly doped with silicon at 1xc3x971016 cmxe2x88x923. Immediately below collector 130 is subcollector 110, which is also gallium-arsenide. However, subcollector 110 is doped with silicon at a significantly higher concentration, typically in the range of 5xc3x971018 cmxe2x88x923. In GaAs HBT 100, collector layer 130 can be between 0.3 microns and 2.0 microns thick, and subcollector 110 can be between 0.3 microns and 2.0 microns thick.
As discussed above, when an NPN gallium-arsenide HBT is in the active mode, the depletion region at the collector-base junction is widened due to the electric field generated by the applied voltage. Thus, when voltage is applied to GaAs HBT 100, the narrow depletion region that formed originally at the P-N junction between base 112 and collector 130, widens. Further, as the collector voltage is gradually increased, the edge of the depletion region extends deeper into collector 130, and at a high enough voltage, the depletion region eventually reaches subcollector 110. It is known, however, that due to the significantly higher doping level of subcollector 110, the process of depleting subcollector 110 occurs much slower. Depending on the thickness and doping level of collector 130, breakdown voltage may occur by the time the edge of the depletion region nears the interface between collector 130 and subcollector 110, or after the depletion region encroaches into subcollector 110.
As a result of the depletion region extending through collector 130 and reaching the collector-subcollector interface, a strong electric field develops near heavily doped subcollector 110. The strong electric field initiates filamentation which, stated simply, is localized current that causes high power dissipation in a small area with a correspondingly high increase in the localized temperature within the device. It has been theorized that filamentation arises, for example, through localized inhomogeneties in the subcollector region from the precipitation of the dopant, e.g. silicon, in the heavily doped subcollector, through natural defects in the crystal, or perhaps through internal thermal gradients in the device. Regardless of the precise mechanism of filamentation, the end result is premature device failure.
There is thus a need in the ail for method for fabricating a gallium-arsenide HBT that minimizes the likelihood of filamentation so as to enhance the operating condition of the device.
The present invention is directed to method and structure for a heterojunction bipolar transistor. In one embodiment, the invention results in a heterojunction bipolar transistor (xe2x80x9cHBTxe2x80x9d) with a collector that prevents the depletion region from reaching near the subcollector, thereby preventing the formation of a strong electric field at the junction between the collector and the subcollector and thereby inhibiting filamentation in the subcollector.
According to one embodiment of the invention, a heavily doped subcollector is formed. The heavily doped subcollector can comprise, for example, gallium-arsenide doped with silicon at a high concentration. Subsequently, a collector is fabricated over the heavily-doped subcollector, wherein the collector comprises a medium-doped collector layer adjacent to the subcollector and a low-doped collector layer over the medium-doped collector layer. Both the medium-doped collector layer and the low-doped collector layer can comprise gallium-arsenide doped with silicon, for example, at between approximately 5xc3x971016 cmxe2x88x923 and approximately 1xc3x971018 cmxe2x88x923 in the case of the medium-doped collector layer, and at between approximately 1xc3x971016 cmxe2x88x923 and approximately 3xc3x971016 cmxe2x88x923 in the case of the low-doped collector layer.
According to one embodiment, the collector can further comprise a medium/high-doped collector layer over the low-doped collector layer, and the medium/high-doped collector layer can comprise gallium-arsenide doped with silicon at between approximately 2xc3x971016 cmxe2x88x923 and approximately 3xc3x971018 cmxe2x88x923. Thereafter, a base is grown over the collector, and an emitter is deposited over the base. The collector of the HBT prevents the depletion region from reaching the subcollector without unduly impeding the expansion of the depletion region. As a result, filamentation in the subcollector is prevented, but the HBT""s performance may be optimized for particular circuits.
Further, a HBT structure can be fabricated in which the collector comprises a medium-doped collector layer adjacent to a heavily doped subcollector. The collector also comprises a low-doped collector layer over the medium-doped collector layer. The HBT further comprises a base over the low-doped collector layer and an emitter over the base. The result is a HBT structure wherein the depletion region is impeded from reaching the subcollector, and filamentation is prevented from initiating in the subcollector.